Current video encoders (MPEG, H.264, etc) use a representation by blocks of the video sequence. The images are split into macroblocks, each macroblock is itself split into blocks and each block, or macroblock, is encoded by intra image or inter image prediction. Thus, some images are encoded by spatial prediction (intra prediction), while other images are encoded by temporal prediction (inter prediction) with respect to one or more reference encoded-decoded images, with the aid of a motion compensation known by the person skilled in the art.
For each block, there is encoded a residual block, also called prediction residue, corresponding to the original block reduced by a prediction. The residual blocks are transformed by a transform of the discrete cosine transform (DCT) type, then quantized using, for example, a scalar type quantization. Coefficients, some of which are positive and others negative, are obtained at the end of the quantization step. They are then scanned in a generally zigzag read order (as in the JPEG standard), thereby providing for exploiting the significant number of zero coefficients in the high frequencies. At the end of the aforementioned scan, a one-dimensional list of coefficients is obtained, which will be called “quantized residue”. The coefficients of this list are then encoded by an entropy encoding.
The entropy encoding (for example of the arithmetic coding or Huffman coding type) is carried out as follows:                an information item is entropically encoded in order to indicate the position of the last non-zero coefficient in the list,        for each coefficient located before the last non-zero coefficient, an information item is entropically encoded in order to indicate whether or not the coefficient is zero,        for each non-zero coefficient indicated previously, an information item is entropically encoded in order to indicate whether or not the coefficient is equal to one,        for each coefficient that is not zero and not equal to one, and located before the last non-zero coefficient, an amplitude information item (absolute value of the coefficient, which value is reduced by two) is entropically encoded,        for each non-zero coefficient, the sign assigned to it is encoded by a ‘0’ (for the + sign) or a ‘1’ (for the − sign).        
According to the H.264 technique for example, when a macroblock is split into blocks, a data signal, corresponding to each block, is transmitted to the decoder. Such a signal comprises:                the quantized residues contained in the abovementioned list,        information items representative of the encoding mode used, in particular:                    the prediction mode (intra prediction, inter prediction, default prediction producing a prediction for which no information is transmitted to the decoder (called “skip”);            information specifying the type of prediction (orientation, reference image, etc);            the type of partitioning;            the type of transform, for example 4×4 DCT, 8×8 DCT, etc:            the motion information if necessary;            etc.                        
The decoding is performed image by image, and for each image, macroblock by macroblock. For each partition of a macroblock, the corresponding elements of the stream are read. The inverse quantization and the inverse transformation of the coefficients of the blocks are performed to produce the decoded prediction residue. Then, the prediction of the partition is calculated and the partition is reconstructed by adding the prediction to the decoded prediction residue.
The intra or inter encoding by competition, as implemented in the H.264 standard, thus relies on placing various encoding information items, such as those mentioned above, in competition with the aim of selecting the best mode, i.e. the one which will optimize the encoding of the partition in question according to a predetermined performance criterion, for example the rate distortion cost that is well known to the person skilled in the art.
The information items representative of the encoding mode selected are contained in the data signal transmitted by the encoder to the decoder. The decoder is thus capable of identifying the encoding mode selected at the encoder, then applying the prediction that conforms with this mode.
The document “Data Hiding of Motion Information in Chroma and Luma Samples for Video Compression”, J.-M. Thiesse, J. Jung and M. Antonini, International workshop on multimedia signal processing, 2011, presents a data hiding method implemented during a video compression.
More specifically, it is proposed to avoid including in the signal to be transmitted to the decoder at least one competition index as issued from a plurality of competition indexes to be transmitted. Such an index is for example the index MVComp which represents an information item for identifying the motion vector predictor used for a block predicted in inter mode. Such an index which can take the value 0 or 1 is not included directly in the signal of encoded data items, but transported by the parity of the sum of the coefficients of the quantized residue. An association is created between the parity of the quantized residue and the index MVComp. By way of example, an even value of the quantized residue is associated with the index MVComp of value 0, while an odd value of the quantized residue is associated with the index MVComp of value 1. Two cases can arise. In a first case, if the parity of the quantized residue already corresponds to that of the index MVComp which is desired to be transmitted, the quantized residue is encoded conventionally. In a second case, if the parity of the quantized residue is different from that of the index MVComp which is desired to be transmitted, the quantized residue is modified such that its parity is the same as that of the index MVComp. Such a modification involves incrementing or decrementing one or more coefficients of the quantized residue by an odd value (e.g. +1, −1, +3, −3, +5, −5 etc) and retaining only the modification which optimizes a predetermined criterion, in this case the previously mentioned rate distortion cost.
At the decoder, the index MVComp is not read in the signal. The decoder is satisfied simply with conventionally determining the residue. If the value of this residue is even, the index MVComp is set to 0. If the value of this residue is odd, the index MVComp is set to 1.
In accordance with the technique that has just been presented, the coefficients which undergo the modification are not always chosen optimally, such that the modification applied brings about disturbances in the signal transmitted to the decoder. Such disturbances are inevitably detrimental to the effectiveness of the video compression.